As mentioned above, a larger tank is more conducive to having a community with crayfish included. However, never be surprised if/when you find fish missing.
Also if I were you, careful about Bristle worms, they often hide in live rock. And they will bite.
1) 68 degrees is OK for LR.
2) As long as the heater is fully submerged, you can turn it on.
3) I handle LR with bare hands all the time
Who else lives with the guppies? White clouds might eat fry. Most fish will eat the fry if they get a chance. Otherwise you should see fry about every
If the bottom of your aquarium is glass, just attach the mirror to the underside of the aquarium, then you don't have to worry about mirror compatibility